This invention relates to new polymeric materials especially suitable for use in biomedical applications, and processes for their production and use.
According to the present invention there is provided a crosslinkable polymer formed from monomers including
a) a hydroxyl-group containing monomer of the general formula I 
in which R26 is hydrogen or C1-4-alkyl group A1 is xe2x80x94Oxe2x80x94 or xe2x80x94NR27xe2x80x94 where R27 is hydrogen or a C1-4-alkyl group or a group B9Rxe2x80x94(OH)n;
B9 is a bond, a straight or branched alkylene, a oxa alkylene or oligooxa alkylene group;
R13 is a n+1 functional (optionally substituted) C1-24 alkylene group,
n is an integer of one or more; and
b) a reactive monomer of the general formula (II) 
in which R19 is hydrogen or a C1-4-alkyl group or A2 is xe2x80x94Oxe2x80x94 or xe2x80x94NR21xe2x80x94 where R21 is hydrogen, or a C1-4-alkyl group R12 is C1-24 straight or branched alkylene, oxaalkylene or oligo oxaalkylene group in which each alkylene group has 1 to 6 carbon atoms
A3 is a band or xe2x80x94Oxe2x80x94
each R16 independently selected from C1-6-alkyl groups.
In hydroxyl group containing monomer of the general formula I, R26 is preferably hydrogen or methyl, most preferably methyl. A1 is preferably xe2x80x94Oxe2x80x94. B9 is preferably a bond.
R13 is preferably selected from C2-6-alkylene groups, most preferably C2-4-alkylene, most preferably propylene. The monomer I may consist of a mixture of compounds having different groups R13. Preferably, in such a mixture, all groups R13 have the same number of carbon atoms, but may comprise a mixture of different isomers. For instance where the group R13 is propylene, it may consist of a mixture of 1,2-propylene and 1,3-propylene.
In the reactive monomer of the general formula II, R19 is preferably selected from hydrogen and methyl groups, most preferably R19 is methyl. A2 is preferably xe2x80x94Oxe2x80x94. A3 is preferably xe2x80x94Oxe2x80x94. R12 is preferably C2-6-alkylene, preferably C2-4-alkylene. Again, the monomer of the formula II may comprise a mixture of compounds having different groups R12.
R16 is selected from C1-6-alkyl groups, preferably C1-2-alkyl groups.
Preferred polymers are made from monomers including a zwitterionic monomer of the general formula III 
wherein B is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene chain optionally containing one or more fluorine atoms up to and including perfluorinated chains or, if X contains a carbonxe2x80x94carbon chain between B and the zwitterionic moiety or if Y contains a terminal carbon atom bonded to B, a valence bond;
X is a zwitterionic group
R is hydrogen or a C1-C4 alkyl group; and
A is xe2x80x94Oxe2x80x94 or xe2x80x94NR1xe2x80x94 where R1 is hydrogen or a C1-C4 alkyl group or R1 is xe2x80x94Bxe2x80x94X where B and X are as defined above.
In compounds of formula (III) it is preferred that K and B contain up to 12 carbon atoms in total.
In the zwitterionic monomer of the general formula III, R is preferably selected from hydrogen and methyl, and is most preferably methyl.
Preferably B9 is an alkylene group of formula xe2x80x94(CR32)axe2x80x94, wherein the groups xe2x80x94(CR32)xe2x80x94 are the same or different, and in each group xe2x80x94(CR32)xe2x80x94 the groups R3 are the same or different and each group R3 is hydrogen, fluorine or C1-4 alkyl or fluoroalkyl, preferably hydrogen, and a is from 1 to 12, preferably 1 to 6;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR42)bO]c(CR42)bxe2x80x94 where the groups xe2x80x94(CR42)xe2x80x94 are the same or different and in each group xe2x80x94(CR42)xe2x80x94 the groups R4 are the same or different and each group R4 is hydrogen, fluorine or C1-4 alkyl or fluoroalkyl, preferably hydrogen, and b is from 1 to 6, preferably 2 or 3 and c is from 2 to 11, preferably 2 to 5; or
if X contains a carbonxe2x80x94carbon chain between B and the centre of permanent positive charge or if Y contains a terminal carbon atom, a valence bond.
Preferred groups B include alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 12 carbon atoms optionally containing one or more fluorine atoms.
Preferred zwitterionic groups on zwitterionic monomer III are groups in which the cationic moiety is based on a quaternary ammonium group and the anionic moiety is based on a phosphate group. Preferred zwitterionic groups are ammonium phosphate ester zwitterionic groups. Preferably the ammonium is quaternary ammonium and the phosphate group is a diester group, or amide analogue. Usually the cationic group is located at the end of pendant group X distant from B.
Preferably X is a group of formula 
in which the moieties X1 and X2, which are the same or different, are xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94 or a valence bond, preferably xe2x80x94Oxe2x80x94, and W+ is a group comprising an ammonium, phosphonium or sulphonium cationic group and a group linking the anionic and cationic moieties which is preferably a C1-12-alkylene group.
Preferably W contains as cationic group an ammonium group, more preferably a quaternary ammonium group.
The group W+ may for example be a group of formula
xe2x80x94W1xe2x80x94N+R233, xe2x80x94W1xe2x80x94P+R23a3, xe2x80x94W1xe2x80x94S+R23a2 or xe2x80x94W1-Het+ in which:
W1 is alkylene of 1 or more, preferably 2-6 carbon atoms optionally containing one or more ethylenically unsaturated double or triple bonds, disubstituted-aryl, alkylene aryl, aryl alkylene, or alkylene aryl alkylene, disubstituted cycloalkyl, alkylene cycloalkyl, cycloalkyl alkylene or alkylene cycloalkyl alkylene, which group W1 optionally contains one or more fluorine substituents and/or one or more functional groups; and either
the groups R23 are the same or different and each is hydrogen or alkyl of 1 to 4 carbon atoms, preferably methyl, or aryl, such as phenyl or two of the groups R23 together with the nitrogen atom to which they are attached form a heterocyclic ring containing from 5 to 7 atoms or the three groups R23 together with the nitrogen atom to which they are attached form a fused ring structure containing from 5 to 7 atoms in each ring, and optionally one or more of the groups R23 is substituted by a hydrophilic functional group, and
the groups R23a are the same or different and each is R23 or a group OR23, where R23 is as defined above; or
Het is an aromatic nitrogen-, phosphorus- or sulphur-, preferably nitrogen-, containing ring, for example pyridine.
Preferably W1 is a straight-chain alkylene group, most preferably 1,2-ethylene.
Most preferred groups are the groups of formula (IVA), (IVB), (IVC), (IVD) and (IVE) as defined below: monomers containing such groups may be used alone or in combination with further zwitterionic monomers to provide a copolymer. Of these groups (IVB) are particularly preferred.
The groups of formula (IVA) are: 
where the groups R6 are the same or different and each is hydrogen or C1-4 alkyl and d is from 2 to 4.
Preferably the groups R6 are the same. It is also preferable that at least one of the groups R6 is methyl, and more preferable that the groups R6 are both methyl.
Preferably d is 2 or 3, more preferably 3.
When X is a group of formula (IVA) preferably B is a group of formula xe2x80x94(CR32)xe2x80x94 or xe2x80x94(CR32)2xe2x80x94, eg. xe2x80x94(CH2)xe2x80x94 or xe2x80x94(CH2,CH2)xe2x80x94.
The groups of formula (IVB) are: 
were the groups R7 are the same or different and each is hydrogen or C1-4 alkyl, and e is from 1 to 4.
Preferably the groups R7 are the same. It is also preferable that at least one of the groups R7 is methyl, and more preferable that the groups R7 are all methyl.
Preferably e is 2 or 3, more preferably 2.
When X is a group of formula (IVB) preferably B is a group of formula xe2x80x94(CR32)xe2x80x94 or xe2x80x94(CR32)2xe2x80x94, eg. xe2x80x94(CH2)xe2x80x94 or xe2x80x94(CH2CH2)xe2x80x94.
The groups of formula (IVC) are: 
wherein the groups R8 are the same or different and each is hydrogen or C1-4 alkyl, R8a is hydrogen or, more preferably, a group xe2x80x94C(O)B1R8b where R8b is hydrogen or methyl, preferably methyl, B1 is a valence bond or straight or branched alkylene, oxaalkylene or oligo-oxaalkalyene group, and f is from 1 to 4; and
if B is other than a valence bond Z is 1 and if B is a valence bond Z is O, if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.
Preferably the groups R8 are the same. It is also preferable that at least one of the groups R8 is methyl, and more preferable that the groups R8 are all methyl.
Preferably f is 1 or 2, more preferably 2.
Preferably B1 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3a2)aaxe2x80x94, wherein the groups xe2x80x94(CR3a2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3a2)xe2x80x94 the groups R3a are the same or different and each group R3a is hydrogen or C1-4 alkyl, preferably hydrogen, and is from 1 to 12, preferably 1 to 6;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4a2)baO]caxe2x80x94 where the groups xe2x80x94(CR4a2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4a2)xe2x80x94 the groups R4a are the same or different and each group R4a is hydrogen or C1-4 alkyl, preferably hydrogen, and ba is from 1 to 6, preferably 2 or 3, and ca is from 1 to 12, preferably 1 to 6.
Preferred groups B1 include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 12 carbon atoms.
Preferably B and B1 are the same.
When X is a group of formula (IVC) preferably B is a group of formula xe2x80x94[(CR42CR42)cOb]CR42CR42xe2x80x94, eg. xe2x80x94(CH2CH2O)c(CH2CH2)xe2x80x94.
The groups of formula (IVD) are: 
wherein the groups R9 are the same or different and each is hydrogen or C1-C4 alkyl, R9a is a hydrogen or, more preferably, a group xe2x80x94C(O)B2R9b, R9b is hydrogen or methyl, preferably methyl, B2 is a valence bond or a straight or branched alkylene, oxaalkylene or oligoxe2x80x94oxaalkylene group, and g is from 1 to 4; and
if B is other than a valence bond Z is 1 and if B is a valence bond Z is O if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.
Preferably the groups R9 are the same. It is also preferable that at least one of the groups R9 is methyl, and more preferable that the groups R9 are all methyl.
Preferably g is 1 or 2, more preferably 2.
Preferably B2 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3b2)abxe2x80x94, wherein the groups xe2x80x94(CR3b2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3b2)xe2x80x94 the groups R3b are the same of different and each group R3b is hydrogen or C1-4 alkyl, preferably hydrogen, and ab is from 1 to 12, preferably 1 to 6;
an oxaalkylene group such as alkoxyalkyl having 1 to 6, carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4b2)bbO]cbxe2x80x94 where the groups xe2x80x94(CR4b2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4b2)xe2x80x94 the groups R4b are the same or different and each group R4b is hydrogen or C1-4 alkyl, preferably hydrogen, and bb is from 1 to 6, preferably 2 or 3, and cb is from 1 to 12, preferably 1 to 6.
Preferred groups B2 include a valence bond and alkylene, oxalkylene and oligo-oxalkylene groups of up to 12 carbon atoms.
Preferably B and B2 are the same.
When X is a group of formula (IVD) preferably B is a group of formula xe2x80x94[(CR42CR42)bO]cCR42CR42xe2x80x94, eg. xe2x80x94(CH2CH2O)cCH2CH2xe2x80x94.
The groups of formula (IVE) are: 
wherein the groups R10 are the same or different and each is hydrogen or C1-4 alkyl, R10a is hydrogen or, more preferably, a group xe2x80x94C(O)B3R10b where R10b is hydrogen or methyl, preferably methyl, B3 is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, and h is from 1 to 4; and
if B is other than a valence bond Z is 1 and if B is a valence bond Z is O if X is directly bonded to the oxygen or nitrogen and otherwise Z is 1.
Preferably the groups R10 are the same. It is also preferable that at least one of the groups R10 is methyl, and more preferable that the groups R10 are all methyl.
Preferably h is 1 or 2, more preferably 2.
Preferably B3 is:
a valence bond;
an alkylene group of formula xe2x80x94(CR3c2)acxe2x80x94, wherein the groups xe2x80x94(CR3c2)xe2x80x94 are the same or different, and in each group xe2x80x94(CR3c2)xe2x80x94 the groups R3c are the same or different and each group R3c is hydrogen or C1-4 alkyl, preferably hydrogen, and ac is from 1 to 12, preferably 1 to 6;
an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably xe2x80x94CH2O(CH2)4xe2x80x94; or
an oligo-oxaalkylene group of formula xe2x80x94[(CR4c2)bcO]ccxe2x80x94 where the groups xe2x80x94(CR4c2)xe2x80x94 are the same or different and in each group xe2x80x94(CR4c2)xe2x80x94 the groups R4c are the same or different and each group R4c is hydrogen or C1-4 alkyl, preferably hydrogen, and bc is from 1 to 6, preferably 2 or 3, and cc is from 1 to 12, preferably 1 to 6.
Preferred groups B3 include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 12 carbon atoms.
Preferably B and B3 are the same.
When X is a group of formula (IVE) preferably B is a group of formula xe2x80x94[(CR42CR42)bO]cCR42CR42xe2x80x94, eg. xe2x80x94(CH2CH2O)cCH2CH2xe2x80x94.
A polymer according to the invention is preferably formed from monomers including a comonomer of the formula 
in which R14 is hydrogen or a C1-4-alkyl group
A4 is xe2x80x94Oxe2x80x94 or xe2x80x94NR15xe2x80x94 where R15 is hydrogen, C1-4-alkyl group or a group Q,
and Q is selected from hydroxyl groups
a straight or branched alkyl, alkoxyalkyl or (oligo-alkoxy)alkyl chain containing 1 to 24 carbon atoms unsubstituted or substituted by one or more fluorine atoms and optionally containing one or more carbonxe2x80x94carbon double or triple bonds;
a siloxane group xe2x80x94(CR 16a2)qq(SiR16b2)(OSiR16b2)ppR16b in which each group R16a is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms or aralkyl, for example benzyl or phenethyl, each group R16b is alkyl of 1 to 4 carbon atoms, qq is from 1 to 6 and pp is from 0 to 49; and
C1-24-alkyl groups substituted with a group Q5 selected from anionic groups selected from carboxylate, sulphonate, hydrogenphosphate and phosphate groups and cationic groups selected from xe2x80x94N+R313 in which each group R313, in which each group R31 is the same or different, and is hydrogen or alkyl of 1 to 6 carbon atoms two of which groups R31 may together from a heterocyclic ring containing from 5 to 7 atoms, +N Het where Het is an unsaturated heterocyclic group substituted or unsubstituted by one or more alkyl groups of 1 to 4 carbon atoms, and groups P+R323 in which each group R32 is the same or different and is hydrogen or alkyl of 1 to 6 carbons atoms, two of which groups R32 may together form a heterocyclic ring containing from 5 to 7 atoms.
In the formula V, group R14 is preferably selected from hydrogen and methyl. A4 is preferably xe2x80x94Oxe2x80x94.
In the monomer of formula V, the group Q is most conveniently a hydrophobic group, preferably an alkyl, a fluoroalkyl or a siloxane group.
Preferred monomers V are comonomers in which Q is
a) an alkyl group which group optionally contains one or more etheric oxygen atoms and optionally one or more carbonxe2x80x94carbon double or triple bonds for instance which has 6 or more carbon atoms, or
b) a fluoroalkyl group, preferably of 6 or more carbon atoms, which group optionally contains one or more etheric oxygen atoms and optionally one or more carbonxe2x80x94carbon double or triple bonds, or
c) a siloxane group, containing up to 50 silicon atoms, preferably in a linear chain.
Preferably the alkyl or fluoroalkyl groups Q contains up to 24 carbon atoms, for instance up to 18 carbon atoms. For instance in the compound of the formula V, Q is a straight or branched alkyl, alkoxyalkyl or oligoalkoxyalkyl chain containing 1-24 carbon atoms, unsubstituted or substituted by one or more fluorine atoms and optionally containing one or more carbonxe2x80x94carbon double or triple bonds or is a siloxane group as defined above.
Most preferably Q is:
an alkyl group of formula xe2x80x94(CR172)mCR173, wherein the groups xe2x80x94(CR172)xe2x80x94 are the same or different, and in each group xe2x80x94(CR172)xe2x80x94 the groups R17are the same or different and each group R17 is hydrogen, fluorine or C1-4 alkyl or fluoroalkyl and m is from 5 to 23 if Q contains no fluorine atoms or from 1 to 23, preferably 5 to 23, if Q contains one or more fluorine atoms;
an alkoxyalkyl having 1 to 12 carbon atoms in each alkyl moiety; unsubstituted or substituted by one or more fluorine atoms; or
an (oligo-alkoxyl)alkyl group of formula xe2x80x94[(CR182)nO]o(CR182)nR18 where the groups xe2x80x94(CR182)xe2x80x94 are the same or different and in each group xe2x80x94(CR182)xe2x80x94 the groups R18 are the same or different and each group R18 is hydrogen, fluorine or C1-4 alkyl or fluoroalkyl and n is from 2 to 6, preferably 3 to 4, and o is from 1 to 12.
Particularly preferred groups Q are straight chain alkyl or fluoroalkyl groups optionally containing one or more carbonxe2x80x94carbon double or triple bonds.
Where Q is a siloxane group as defined above, each group xe2x80x94(CR16a2)xe2x80x94 may be the same or different, preferably the same, and preferably each group R16a is hydrogen. Preferably qq is from 2 to 4, and is most preferably 3. Each group xe2x80x94(SiR16b2)xe2x80x94 may be the same or different, preferably the same, and preferably each group R16b is methyl. Preferably pp is from 4 to 29.
In one specific embodiment the group Q does not contain any ethylenic unsaturation, i.e. any carbonxe2x80x94carbon double or triple bonds.
Particular examples of comonomers containing an alkyl, fluoroalkyl or siloxane group include: methylmethacrylate, butylmethacrylate, n-dodecyl methacrylate, octadecyl methacrylate, hexadecyl methacrylate, 1H,1H,2H,2H-heptadecafluorodecyl methacrylate, p-octyl styrene, p-dodecyl styrene and monomethacryloxypropyl terminated siloxanes especially poly(dimethyl siloxanes). n-Dodecyl methacrylate is particularly preferred.
In the novel polymers, it is preferred that all of the groups R26, R19, R and R14 represent the same meaning, preferably methyl. Preferably also the groups A1, A2, A and A4 represent the same group, and are preferably, xe2x80x94Oxe2x80x94. Where such common features are present in the monomers, it is believed that copolymerisation proceeds in an optimum fashion, since the monomers are likely to have similar reactivity ratios.
The polymers are made from their constituent monomers by radical polymerisation, typically using thermal, redox or photochemical initiation. The polymerisation conditions could be such that reaction between the silyl group of monomer of the formula II with the hydroxyl group of the monomer of the formula I does not take place to any significant extent during polymerisation.
For thermal polymerisation a temperature from 40 to 100xc2x0 C., typically 50 to 80xc2x0 C. is used. For photochemical polymerisation actinic radiation such as gamma, U.V., visible, or microwave radiation may be used. Typically U.V. radiation of wavelength 200 to 400 nm is used.
The polymerisation is generally performed in a reaction medium, which is for instance a solution or dispersion of the monomers in a liquid phase using as a solvent for example acetonitrile, dimethyl formamide, chloroform, dichloromethane, ethyl acetate, dimethyl sulphoxide, dioxan, benzene, toluene, tetrahydrofuran, or where the polymer does not contain groups which react with protic solvents, water or an alkanol containing from 1 to 4 carbon atoms, e.g. methanol, ethanol or propan-2-ol. Alternatively, a mixture of any of the above solvents may be used.
The polymerisation may be carried out in the presence of one or more polymerisation initiators, usually free radical generators, usually peroxides or azo initiators, such as benzoyl peroxide, 2,2xe2x80x2-azo-bis(2-methylpropionitrile) or benzoin methyl ether. Other polymerisation initiators which may be used are disclosed in xe2x80x9cPolymer Handbookxe2x80x9d, 3rd edition, Ed. J. Brandrup and E. H. Immergut, Pub. Wiley-Interscience, New York, 1989.
Generally the polymerisation is performed for 1 to 72 hours, preferably 8 to 48, for instance 16 to 24 hours, and under an inert atmosphere of for example nitrogen or argon.
The polymer is generally purified by dialysis, precipitation in a nonxe2x80x94solvent (e.g. diethyl ether or acetone) or ultrafiltration. The resulting polymer is generally dried under vacuum, eg. for 5 to 72 hours and has a molecular weight from 10,000 to 10 million, preferably from 20,000 to 1 million.
It is preferable for the monomer of the formula I to be present in a molar excess to the monomer of the formula II. The two monomers are generally provided in the polymer in amounts so as to give the desired level of crosslinking when the polymer is used. Preferably the molar ratio of the monomer of the formula I to the monomer of the formula II is in the range 1:2 to 10:1, preferably 1:1 to 10:1, most preferably 2:1 to 6:1.
Where a zwitterionic monomer of the formula III is included, it is generally contained in a molar quantity in the range 0.01%, preferably at least 1%, most preferably at least 5%, for instance at least 10%. The total amount is preferably less than 50%, for instance in the range 15 to 33%.
Where comonomer of the formula V is included in the polymer, it may be present in an amount in the range 1 to 99% by mole, most preferably in an amount in the range 1 to 95%, most preferably in the range 10 to 90%, for instance in the range 25 to 80%.
The amount of monomer of the formula II is generally at least 1%, preferably at least 2%, for instance in the range 3 to 10%. The amount of monomer of the formula I is generally higher than the molar amount of monomer of the formula II, since this tends to encourage complete reaction of the trialkoxy silyl group of the monomer of the formula II. The residual, uncrosslinked hydroxyl group of the monomer of formula I is not very reactive, so that its presence in the crosslinked product does not cause problems with reactive interactions with fluids or solid substrates with which it is in contact.
The crosslinkable polymer is of particular utility for coating applications. The present invention provides further coating compositions containing the crosslinkable polymer. Such compositions generally comprise a liquid vehicle within which the polymer is dissolved or dispersed. Preferably the liquid vehicle includes a solvent for the polymer in which the polymer is dissolved. For instance the solvent may be an aqueous based solvent, but is more likely to be an organic solvent, for instance an alcohol or ether solvent. Coating compositions may comprise other components which assist wetting of an underlying surface and/or coating integrity.
The present invention provides also processes in which substrates are coated with a liquid coating composition according to the invention, followed by a step in which solvent is removed from the coating, for instance by evaporation, and in which the polymer is, after coating, subjected to a crosslinking step in which it is subjected to conditions in which crosslinking takes place between the hydroxyl groups of the monomer of the formula I and one of the said OR16 groups of monomer of the formula II. Such reaction conditions generally involve subjecting the polymer to raise temperature, for instance under reduced pressure to remove alcohol R16OH by-product.
For some coating processes, it may be desirable to polymerise the starting monomers and crosslink in a single step. In this case, a coating composition comprises monomers and, if necessary, a solvent for the monomers, and is subjected to conditions whereby reaction between the trialkoxysilyl group of monomer II and the hydroxyl group of monomer I takes place simultaneously with radical initiated polymerisation.
The use of monomers of the formula I in combination with the formula II gives very good crosslinking levels, whilst producing a polymer product which has good integrity, that is cohesion, but adequate flexibility. It therefore provides a good coating for use on compliant substrates, especially substrates which are subjected to lengthening or other dimensional change in use. For instance the substrate which is coated may be the metal of a stent or other biomedical device which is expanded in use. The substrate may be an elastomer, or a plastically expandable plastics material.